Does an upgrade make sense?

Subs are rarely a bad idea, and are usually well received.  

It looks like you've picked most of the low hanging fruit, but assuming your speakers are well designed with good quality drivers, but built to a price point, it's very possible the crossover components are ripe for an upgrade.  Upgrading the caps and cheap cast resistors to non-inductive wire wound types can be fairly easy, fairly cheap, and fairly audible.  It won't change the overall character of the speaker, but could definitely improve clarity (and things associated with that....ie: soundstage, etc.)

Assuming you've done what you can, or are willing, to address room acoustic then I also would say a DAC should be your next consideration. And I'd look at spending as much as you can, I don't think a <$1000 DAC is going to make much difference but maybe some used options. 

FWIW, drill down your set up first. You can't really judge the contributions for a proposed DAC until you have that right (unless you want to build your entire system around a DAC, which sounds dumb to me). Re set up, ultimately what I would do if I had a room that size would be to drive the mains full range and set up two subs placed to smooth out the bass (usually where you are sitting) caused by nulls. If I wanted to fill the room with sound on the cheap, I'd just set up an old fashioned Hafler  system (using 2 separate satellites and a attenuator, and if you are semi anal about your main systems sound a seperate amp) There are so many trade offs.............:-)

 we have been selling dali loudspeakers for over 20 years 

personally we would recommend upgrading to the newer Dali Rubicons

the mentor series was a good product but lacked the magic of the more expensive helicons and the newer rubicons sounded even better then the older series

the rubicons are far better in every way.

the rest of your system is comprised of solid components.

Dave and Troy

audio Intellect NJ.

Dali dealers

LOL all good advice and pulling me in so many directions. I’m thinking I’d update speakers before bringing in subs. Have thought of the Rubicons—I got the Mentors because I couldn’t afford the Epicons at the time. I also see that my DAC is long in the tooth. I’ve done a lot around vibration control and have upgraded power cables. Bottom line— I’m thinking DAC or speakers. More bang for the buck with a new DAC I think. I’d want balanced outputs. What do you recommend $3000 or so, used is fine.